Step 7: Balls to the Wall!
Best for compact cars (or people that have several thousand balls to unload), fill the car up with balls so that, upon opening the door, the balls roll out.
This also works great with styrofoam peanuts — the most economical, but least environmental of the bunch.
Bonus points if you find a way to incorporate a gaggle of white turtledoves to also fly out of the car, gloriously punctuating your prank. Minus points if you kill the doves.